1. Field of the Invention
The present invention generally relates to an image forming apparatus, and more particularly, to an image forming apparatus such as a laser printer and an LED printer that can print images at a print resolution, wherein the resolution of input image data is lower than the print resolution.
2. Description of the Related Art
The print resolution of an image forming apparatus (a laser printer and an LED printer, for example) is typically 600 dpi or 1200 dpi, where the print resolution is the resolution of images that the image forming apparatus can print on paper, for example. The image forming apparatus, however, needs to handle input image data of which resolution is lower than its print resolution. For example, when the image forming apparatus prints a facsimile message, the image forming apparatus needs to handle input image data of about 400 dpi, or 16 lines/mm. That is, the image forming apparatus needs to print 600 dpi images based on 400 dpi input image data without enlargement. The image forming apparatus may need to print 600 dpi images based on 200 or 240 dpi input image data too.
Japanese Patent Laid-Open Application No. 8-108572 discloses a laser printer that can accept multiple-resolution input image data. The laser printer adjusts the resolution in the main scan directions by changing a pixel clock determined by a PLL frequency synthesizer, for example, and adjusts the resolution in the sub scan directions by changing the process linear speed and/or the rotative speed of a polygon motor.
If the print resolution (600 dpi, for example) is an integer multiple of the resolution of input image data (200 dpi, for example), the same input image data may be repeated for multiple times.
Japanese Patent Laid-Open Application No. 2002-23087 discloses a multi-beam laser printer that adjusts resolution in the sub scan directions by changing beam pitch in the sub scan directions.
Japanese Patent Laid-Open Application No. 5-58162 discloses a printer that print 300 dpi images based on 200 dpi input image data by repeating every other dot of the input image data.
If the resolution of the input image data (input resolution) is 400 dpi while the print resolution the image forming apparatus is 600 dpi, that is the print resolution is not an integer multiple of the input resolution, the adjustment of resolution may be made by changing the pixel clock, the process linear speed, the rotation of the polygon motor, or the beam pitch (if multi-beam). The adjustment of resolution may need additional elements and affect the cost of the image forming apparatus.
The changing of the pixel clock requires a PLL frequency synthesizer that is available as a discrete IC chip or a part of an ASIC. The cost of the PLL frequency synthesizer is relatively low. The changing of the pixel clock is effective only for adjusting the resolution in the main scan directions. The resolution in the sub scan directions cannot be adjusted by changing the pixel clock.
The process linear speed is the speed of transported paper and a photosensitive unit. If the process linear speed of a 600 dpi printer is increased up to 3/2 times its original process linear speed, for example, the printer may print 400 dpi images. However, it is not easy to increase the process linear speed of a printer since the printer is optimized at its original process linear speed.
The process linear speed may be reduced. If the process linear speed of a 600 dpi printer is reduced down to 3/4 times its original process linear speed, for example, the resolution in the sub scan directions becomes 800 dpi. If a 600 dpi printer is operated at 1/2 of its original process linear speed, the 600 dpi printer can print 1200 dpi images. If the same input image data are printed twice as described above, the image becomes 400 dpi in the sub scan directions. However, the print speed of the printer is also reduced and the performance of the printer is degraded in this case. The reduction of the process linear speed may be achieved by using a variable speed motor, changing process conditions, and adjusting laser power. The printer needs additional mechanisms for changing its process linear speed.
If the rotative speed of a polygon motor needs to be changed, a variable rotative speed polygon motor available in the market can satisfy the requirement. If the rotative speed of the polygon motor is reduced to 2/3 of its original rotative speed, the resolution in the sub scan directions becomes 2/3 of an original resolution. If the resolutions in both the main scan directions and the sub scan directions need to be reduced from 600 dpi to 400 dpi, the rotative speed of the polygon motor needs to be reduced to 2/3 times its original rotative speed, and the pixel clock needs to be reduced to 4/9 times (2/3 squared) its original frequency. This change is relatively easy, but it takes at least several seconds as switching time to change the rotative speed of the polygon motor. The switching time degrades the performance of the printer.
A multi-beam printer additionally requires a mechanism to switch the beam pitch, which incurs additional cost. It is known that the beam pitch is adjustable by rotating a laser unit.
According to the above consideration, it is recommended that the resolution be electronically adjusted by repeating every other dot of the input image data as described in Japanese Patent Laid-Open Application 5-68162. The drawback of this technique is that dot size corresponding to the same input image data oscillates every other dot. The oscillation of the dot size distorts printed images and degrades their quality.